In a wireless communication system, a base station may allocate a radio resource of an uplink transmission to a terminal. In this case, the radio resource generally includes an uplink (UL) grant information form. And, this grant information may be transmitted to a terminal from a base station via a broadcast/unicast message or a control channel.
When a resource is allocated to a terminal, a base station is able to assign a modulation and coding scheme (hereinafter abbreviated MCS) and a multi-input multi-output (MIMO) scheme, which may differ in accordance with a current radio channel status of the terminal. This method may be called link adaptation. And, the link adaptation scheme may be able to raise spectral efficiency.
Yet, in order for a base station to guarantee a successful transmission despite having no information on a channel of a terminal, the base station may preferably assign a most reliable MCS to the terminal. As mentioned in the above description, a random access procedure can become one example that a base station allocates a resource to a terminal without having information on a channel of the terminal. If a resource is allocated to a terminal in an initial access procedure without information on a transmission power (Tx power) of the terminal, it may become another example of resource allocation.
In the following description, one example of an initial access procedure in IEEE 802.16m system is explained with reference to FIG. 1.
In FIG. 1, assume that a terminal (e.g., advanced mobile station: AMS) matches a downlink (DL) synchronization with a base station (ABS). And, assume that the terminal receives system information and then obtains random access region (or initial ranging region) information.
First of all, a terminal randomly selects one of initial ranging codes and may be then able to transmit the randomly selected initial ranging code to a random access region of a base station [S101].
The base station determines a received state of the corresponding code. If such a physical parameter as a timing and a transmission power needs to be modified, the base station sets a ranging status of a ranging acknowledgement (AAI_RNG-ACK) message to ‘continue’ and then sends the corresponding message to the terminal in a manner that a physical parameter compensation value is contained in the corresponding message [S102].
Subsequently, the terminal retransmits the initial ranging code to the base station by applying the physical parameter compensation value contained in the ranging acknowledgement (ACK) to the corresponding initial ranging code [S103].
If the retransmitted ranging code is normally received, the base station sets a ranging status of the ranging ACK message to ‘success’ and then transmits it to the terminal [S104].
Thereafter, in order for the terminal to send a ranging request (AAI_RNG-REQ) message, the base station is able to transmit allocation information on an uplink resource to the terminal [S105].
The terminal sends the ranging request message to the base station via the uplink resource indicated by the allocation information [S106]. In response to the ranging request message, the base station sends a ranging response (AAI_RNG-RSP) message [S107].
In this case, even if a channel status of a terminal is actually good, a base station allocates MCS (e.g., lowest MCS), which is as low as possible, to the terminal (S105 in case of FIG. 1), whereby resources may be wasted.
Yet, an allocation size of an uplink resource, which is allocated by a base station to enable a terminal to send a prescribed message to the base station, is variable. In accordance with this allocation size, MCS applied to a message to be sent via the corresponding resource is different. And, a lowest MCS may be different for each message. Since a lowest MCS may be set different for each base station, the base station should inform a terminal of a lowest MCS applied to each message. However, in the above-mentioned situation, if a terminal is informed of MCS each time, signaling overhead is increased. Hence, it is preferably necessary to define an efficient method for a terminal to determine a lowest MCS for transmitting a corresponding message.